In a shell-and-tube heat exchanger control loop, the inlet temperature of the heating/cooling utility is primarily treated as which type of process variable?

Introduction / Context:
In heat exchanger temperature control, the process fluid outlet temperature is usually the controlled variable, the utility flow (steam, hot oil, cooling water) is the manipulated variable, and external changes such as utility supply temperature fluctuations are considered loads (disturbances). Recognising variable roles is crucial for designing robust control strategies.

Given Data / Assumptions:

• Controlled variable: process outlet temperature.
• Manipulated variable: utility flow via control valve.
• Utility inlet temperature can vary due to upstream conditions outside the loop.

Concept / Approach:
A “load” in process control is any disturbance that affects the controlled variable but is not directly manipulated by the controller. The inlet temperature of the utility fluid is not typically adjusted by the exchanger’s local controller; it arrives from another system. Thus, changes in that inlet temperature represent a load disturbance that the controller must reject by adjusting the manipulated variable (flow).

Step-by-Step Solution:

Verification / Alternative check:
Standard P&IDs and textbooks classify supply temperature fluctuations as load changes, addressed by feedback or feedforward compensation.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing the utility flow (manipulated) with utility supply temperature (load); both influence heat duty but play different control roles.

Final Answer:
Load variable